A voltage regulator is placed between a power supply and a load circuit for providing a regulated voltage (constant voltage) to the load circuit regardless of fluctuations in the power supply. The voltage regulator can supply the regulated voltage to the load circuit as long as the output voltage of the power supply is greater than the regulated voltage supplied to the load circuit.
A measure of the effectiveness of the voltage regulator is its power supply rejection ratio (PSRR), which is a ratio of amount of noise present on the power supply that is provided to the voltage regulator and the amount of noise which is provided to the load circuit by the voltage regulator. A high PSRR is indicative of a low amount of transmission of noise in the regulated voltage, and a low PSRR is indicative of a high amount of noise transmission in the regulated voltage. A high PSRR, particularly across a wide range of operating frequencies of the devices being supplied by the voltage regulator, is difficult to achieve.
The enormous demand for portable electronic devices such as tablet computers, mobile phones, personal digital assistants (PDAs), and/or portable media players has pushed demand for SoCs (system-on-chip) in which large number of analog and digital circuit are fabricated on a same die. However, these SoCs suffer from noise which arises from sources such as switching of digital circuits, RF blocks and voltage converters.
This noise affects the power supplies through crosstalk and deteriorates the performance of the analog and digital circuits such as PLL, amplifiers and VCO. This in turn, deleteriously impacts critical system specifications like the selectivity of the receiver, spectral purity of the transmitter, and phase error tolerance of digital circuits. Therefore, the voltage regulators are required to safeguard noise-sensitive blocks (analog and digital) from high frequency fluctuations in the power supply. This makes the design of voltage regulators that have a high PSRR (power supply rejection ratio) over a wide frequency range extremely critical for high system performance.